HIV-1 establishes a persistent infection characterized by ongoing viral replication in the face of vigorous host immune responses, CD4 cell depletion and ultimately AIDS. For SIV models of AIDS, attenuated viruses have provided a powerful approach to understand the basis of pathogenicity, as these viruses can be controlled by the host. These models have also provided the best evidence that host immune responses can be elicited that can protect animals from infection and/or disease when challenged with pathogenic isolates. However, for even the best characterized attenuated viruses, the mechanism(s) that underlie their attenuation and the immune correlates of this protection are unknown. Moreover, when protection is achieved, it is typically for homologous rather than heterologous isolates. Our laboratory has shown that mutations in a conserved YxxF trafficking motif in the SIVmac239 Env cytoplasmic tail produce a profoundly attenuated phenotype that renders this highly pathogenic virus susceptible to host immune control. We have also shown that infected animals are protected from infection with a homologous challenge with SIVmac239, and remarkably, are able to control a heterologous challenge virus. We propose to extend our preliminary results in this model to in vitro studies at the cellular and virologic levels and in vivo studies in nonhuman primates to establish the links between cellular effects of the YxxF mutations, attenuation, and host immune control. Four aims are proposed, two for in vitro studies of the cell biology and virology of this model, and two for in vivo studies of pathogenesis and the host immune responses. Aim#1 will characterize interactions of the YxxF and other recently identified TM trafficking signals and identify cellular partners and trafficking pathways on relevant primary cell types. Aim #2 will evaluate the effects of TM tail mutations on virion structure, composition, infectivity, and replicative capacity and explore mechanisms for increased neutralization sensitivity of these viruses noted in our preliminary results. Aim #3 will comprehensively evaluate early and late events of infection in mucosal lymphoid sites during acute infection, determine what cells are infected, what inflammatory responses are elicited, and whether the attenuation observed in this model affects the ability of YxxF mutants to be transmitted mucosally. Aim #4 will evaluate cellular and humoral immune responses to AGY and other TM tail mutants and determine what differences exist compared to SIVmac239 in breadth, specificity and magnitude. Responses will be assessed before and after challenge with a pathogenic heterologous virus to determine what are the immune correlates of heterologous protection. Collectively, our work shows that rationally designed mutations in cellular trafficking signals in Env can be exploited to disrupt the viral/host balance and the outcome of infection and should be a highly informative model for pathogenesis and vaccine research.